It is generally known to use two crane lifting gears at the same time in so-called tandem operation for lifting and lowering heavy and/or long loads. In this case, the load is suspended from two load-receiving means which are each raised or lowered by a dedicated crane lifting gear. The most varied circumstances may cause the load to shift from the horizontal position. In the case of, for example, long goods which are conventionally suspended from the load-receiving means via slinging means such as loops, such a skewed position of the load could lead to the load slipping out of the slinging means. Such dangerous conditions need to be avoided. Conventionally, each of the crane lifting gears can be operated via a control switch. One of the two control switches is then arranged for tandem operation. A change to tandem operation or out of tandem operation is then effected only by using this control switch. In this way, dangerous situations can occur which must be avoided. On the one hand, the change in the type of operation is made without feedback, which means that the operator must trust that tandem operation is activated and, on the other hand, immediate intervention in the operation of the other crane is made by the control switch which is arranged for tandem operation, which can lead to dangerous situations at that location.
On this subject, it is known, for example, from the German laid open document DE 31 47 158 A1 to connect two crane lifting gears for tandem operation through a common control device. This control device is intended to prevent the skewed position described above. To this end, a sensor is disposed on a spreader which is suspended from load-receiving means of both crane lifting gears. By means of this sensor, the position of the spreader is determined. In the event that the spreader leaves its desired horizontal position, tandem operation is exited, one of the two crane lifting gears is stopped and the other crane lifting gear is used to move the spreader back into the horizontal position.
Furthermore, an indoor travelling crane is known from European patent EP 1 380 533 A1, the trolley drive, crane travel drive, lifting gear and control switch of which are connected to one another through a bus. Operational and safety signals are transmitted and received through the bus.
From the German utility model DE 91 15 537 U1 a single crane control system for a single crane and a tandem crane control system for a first crane with a first trolley and for a second crane with a second trolley are already known. For tandem crane control a bidirectional transfer bus is provided, through which, by means of control switches attached thereto, both cranes and their trolleys can be controlled. Controlling a single crane of the two cranes within the tandem crane control system is not described.
Furthermore, the German patent application DE 10 2006 040 782 A1 discloses tandem operation of two cranes within a safety system and separate control of the cranes in single operation. Each of the cranes has an on-board CAN-bus to which a control system is attached. For necessary corrections in tandem operation, the tandem operation is deselected, the correction carried out and tandem operation reselected.